Postmortem studies of human brains have correlated reduced glial cell density with major depression and other psychiatric disorders. Animal studies have not only confirmed the loss of glial cells in animal models of depression but have also suggested that glial cell loss in the prefrontal cortex (PFC) and hippocampus may in fact cause the depressive symptoms. To date, however, there have been no studies that directly demonstrate roles for specific glial cell types in mediating behavioral deficits, nor have the mechanisms that underlie glial cell loss in depression been identified. NG2+ cells were first shown to be important as progenitors of oligodendrocytes. However, NG2+ cells are abundant in the healthy adult brain where myelination is no longer required. Recent work has suggested that NG2+ cells also function in normal brain physiology and homeostasis, forming functional synaptic contacts with neurons and possibly releasing neuroactive factors. Three key preliminary findings form the basis for this proposal. First, we have found that exposing mice to extended behavioral stress dramatically reduces NG2+ cells in the cerebral cortex and hippocampus. Second, we have found that NG2+ cell density is significantly reduced in the PFC of patients diagnosed with depression. Third, using a genetic tool to ablate NG2+ cells in the brain, we have observed that NG2+ cell loss creates cellular, molecular and functional changes similar to those observed in depression, including depressive-like symptoms. Taken together, our findings suggest strong links between external stress and NG2+ viability, and between NG2+ cell function and the maintenance of normal brain physiology and behavior. We hypothesize that NG2+ cells sense the neuronal 'state of health' at neuron-glial synapses and that their loss or atrophy disturbs brain homeostasis and creates depressive disorders. Understanding how NG2+ cells regulate brain homeostasis has the potential to aid development of more effective intervention and prevention strategies for depressive disorders. Using our NG2+ depletion strategy, social defeat stress paradigm (SDSP), and analysis of NG2+ cells in tissue brain samples of subjects with depression, we propose to systematically elucidate the functions performed by NG2+ cells above and beyond their role as oligodendrocyte progenitors and explore their role in the development of depressive-like symptoms.